Phytate and Mineral Absorption: An Updated Perspective
By Mark Messina, PhD, Executive Director, Soy Nutrition Institute
Absorption of minerals from soyfoods is of considerable research interest. Because soyfoods often replace animal foods in the diet, there are questions about how they compare as sources of iron, zinc, and calcium. Milk and cheese are important sources of calcium in American diets, providing more than 40% of intake.1 Beef provides about 25% of zinc in U.S. diets. 2 And while it accounts for only 9% of iron intake (bread and fortified breakfast cereals are the biggest contributors to iron intake) 2 much of the iron in beef is heme iron, which is a highly absorbable form of iron.3 Overall, about 10% to 15% of total iron intake in western populations is heme iron.4
Plant foods contain only nonheme iron, which is far less absorbable. Absorption of nonheme iron is affected by a number of dietary factors. For example, vitamin C is a potent enhancer of nonheme iron absorption. Phytate, the principle storage form of phosphorus in plants, binds to calcium, iron and zinc, reducing their availability.
Although phytate is generally considered an anti-nutrient because of its effects on mineral absorption, its role isn’t limited to these effects. Phytate regulates various cellular functions in plants and may influence many metabolic processes in humans.5 Some research suggests phytate exerts antioxidant and chemopreventive effects.6
In human nutrition, though, most of the focus has been on the relationship of phytate to mineral absorption. Phytate intake can be particularly high in populations that consume diets rich in whole grains and legumes.
For many years, it was believed that there is no adaptation to the inhibitory effects of high-phytate diets. This belief is based on a 1989 study7 which found that the addition of high-phytate bran to wheat rolls inhibited the absorption of iron equally in vegans and non-vegetarians. However, this finding is largely contradicted by research from Iowa State University published in 2015.8 In this study, women followed either a high- or low-phytate diet for eight weeks. At the end of the study, iron absorption from a test meal had improved by 41% in the high-phytate group, suggesting an adaptation to their high phytate intake.
The most likely reason for the different results is that the vegans in the 1989 study were not actually consuming a high-phytate diet. Their diets provided 323 mg of phytate per day. The high-phytate diet in the Iowa State study provided 1,190 mg of phytate.
Another factor may have been the iron status of the study participants. In the older study, there was a wide range of serum ferritin values among the vegans whereas in the Iowa State study, the women had low serum ferritin values. Possibly, adaptation to the inhibitory effects of phytate occurs only among individuals with low iron status. Of course, iron bioavailability is most relevant for those whose iron status is compromised. In any event, the ability to at least partially overcome the inhibitory effects of phytate on iron absorption may be one reason that acute absorption studies tend to exaggerate the effects of both inhibitors and enhancers on iron absorption in comparison to long-term studies.
The Iowa State study didn’t look at effects of phytate on zinc or calcium absorption. This may be an even more important research question since calcium and zinc intakes of those eating plant-based diets tend to be somewhat low. In contrast, people eating plant-based diets often have a high intake of iron, sometimes even surpassing intakes of meat-eaters.
While phytate modestly affects absorption of calcium from soybeans,9 absorption from both calcium-set tofu and fortified soymilk is good and is comparable to cow’s milk. More research is needed, however, on the effects of phytate on zinc absorption from soyfoods.